In one aspect this invention relates to a method of making bisimides having limited solubility in organic solvents. In another aspect, this invention relates to an improved method for the preparation of bis(halophthalimides) monomers useful for the preparation of polyetherimides.
Various types of polyether polymers, such as polyetherimides, polyethersulfones, polyetherketones, and polyethertherketones, have become important as engineering resins by reason of their excellent properties. These polymers are typically prepared by the reaction of dihydroxyaromatic compounds, such as bisphenol A disodium salt, with dihaloaromatic compounds. For example, polyetherimides are conveniently prepared by the reaction of salts of dihydroxyaromatic compounds with bis(halophthalimides).
U.S. Pat. Nos. 5,229,482 and 5,830,974, disclose the preparation of aromatic polyether polymers in relatively non-polar solvents, using a phase transfer catalyst which is substantially stable under the polymerization conditions. Solvents disclosed in U.S. Pat. No. 5,229,482 include o-dichlorobenzene, dichlorotoluene, 1,2,4-trichlorobenzene and diphenyl sulfone. U.S. Pat. No. 5,830,974 discloses the use of solvents such as anisole, diphenylether, and phenetole. Solvents of the same type may be used for the preparation of bis(halophthalimide) intermediates for polyetherimides.
In each of U.S. Pat. Nos. 5,229,482 and 5,830,974 the bis(halophthalimide) is introduced into the polymerization reaction as a substantially pure, isolated compound. This process step is often difficult, since solid bis(halophthalimdes) are typically of very low density and fluffy, making weighing and handling burdensome. By contrast, U.S. Pat. No. 6,235,866 discloses the preparation of a slurry comprising a bis(halophthalimide). The slurry comprising the bis(halophthalimide) is prepared by reacting roughly equimolar quantities of a diamine and a halophthalic anhydride in a solvent to produce the product bis(halophthalimide) and water as a by-product. The slurry comprising the bis(halophthlimide) must be rigorously dried prior to reaction with, for example, bisphenol A disodium salt, to form a product polyetherimide. Producing a dry slurry of the bis(halophthalimide) in a solvent, for example a slurry of bis(4-chlorophthalimide) in ODCB containing less than about 10 ppm water, presents significant challenges. The reaction between the diamine and the halophthalimide is typically dried by distilling a large volume of solvent from the reaction mixture which entrains the by-product water out of the reaction mixture. This method of water removal, although effective as a means of drying the reaction mixture, is inefficient since a relatively large volume of solvent must be distilled from the reaction mixture in order to remove a relatively small amount of water. Reactions wherein by-product water is removed by distillation are characterized by long reaction times, high reaction temperatures, and the use of large volumes of solvent. Under such circumstances, the product bis(halophthalimide) is subject to an increased likelihood of degradation, and the overall process is rendered inefficient as a result of the time and expense required to distill large volumes of solvent. Further, if the diamine has a significant vapor pressure at the imidization reaction temperature (such as m-phenylene diamine), then some of the diamine may be distilled along with the solvent and water byproduct, thus upsetting the reaction stoichiometry and requiring wastewater cleanup. Moreover, when the product bis(halophthalimide) is relatively insoluble the reaction between the diamine and the halophthalic anhydride is characterized by thick phase behavior in which the reaction mixture becomes highly heterogeneous and cannot be agitated effectively. Thick phase behavior is implicated in a variety of undesirable outcomes, such as low reaction rates and low product qualities. The avoidance of thick phase behavior during the preparation of bis(halophthalimides) in particular, and bisimides generally is thus highly desirable.
Thus, there is a need in the art to develop a facile process for the preparation of bis(halophthalimides) having suitable characteristics for conversion to polyetherimide polymers without isolation that overcomes the shortcomings of current synthetic methods. There is also a need to develop processes for the preparation of relatively insoluble bisimdes, such as bis(halophthalimides), which avoids reaction conditions exhibiting thick phase behavior.